1. Field of the Invention
The invention relates to a picture display device for use in a projection color television receiver and comprising: a monochrome picture display tube which is provided at one end with an electron gun and a display screen at the other end, and in which the following coil systems are provided around the envelope of the picture display tube:
a system of deflection coils for deflecting an electron beam in two orthogonal directions, which system comprises a line deflection coil for deflection in the line (horizontal) deflection direction and a field deflection coil for deflection in the field (vertical) deflection direction; and
a system of convergence correction coils.
2. Description of the Related Art
Large-screen color T.V. receivers having screen diagonals of 40 inches or more are commercially available. These receivers generally comprise three separate cathode ray tubes ( red, green and blue, respectively) each with their own system of lenses and whose pictures are projected from the rear onto a viewing screen in such an arrangement that they form a complete colour picture, with the pictures in the three colours being correctly in register. Each of the cathode ray tubes used is provided with a system of deflection coils comprising coaxially arranged line and field deflection coils for scanning the raster in two orthogonal directions and generally also a system of convergence correction coils to enable the three rasters on the viewing screen to register. The system of convergence correction coils is provided between the deflection coil system and the electron gun around the neck of the cathode ray tube. In practice the known coil systems are found to give the following problems:
the raster geometry is satisfactory but the spot quality is poor. An improved spot quality is at the expense of the raster geometry. If in the latter case the raster geometry is to be corrected with the aid of the convergence correction coil system, the power to be applied to this coil system is (prohibitively) large. These problems are even greater if the cathode ray tube has a flat display screen or, which makes the problem even worse, if it has a display screen which is curved towards the vacuum side.